1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor and more specifically, the present invention relates to a method for manufacturing a thin film transistor having a protective layer arranged to protect source and drain metal lines when a substrate upon which the thin film transistor undergoes a cleaning process.
2. Description of the Background Art
FIGS. 1A-1D are cross-sectional views illustrating a process for manufacturing a thin film transistor according to the related art.
Referring to FIG. 1A, source and drain metal lines are formed on an insulating substrate 100. The source metal line is defined by a double-layered laminate of a first source metal line 11S and a second source metal line 12S. The drain metal line is defined by a double-layered laminate of a first drain metal line 11D and a second drain metal line 12D. A low resistance metal such as aluminum (Al) is used to form the first and second source metal lines 11S and 12S. Using a low resistance metal for the source metal lines allows for faster signal transmission in a device where the thin film transistor is used, for example, in a liquid crystal display device.
Referring to FIG. 1B, a buffer layer and an amorphous silicon layer are sequentially deposited to cover the exposed portion of the substrate 100. Thereafter, the amorphous silicon layer is crystallized into a polysilicon layer and then patterned and etched to form an active layer 14. The underlying buffer layer 13 is then etched with the active layer 14 functioning as a mask.
Referring to FIG. 1C, a gate insulating layer and a conductive layer are sequentially deposited on the exposed portion of the substrate 100 and the active layer 14. The conductive layer is patterned and etched to form a gate electrode 16. Next, the gate insulating layer 15 is etched with the gate electrode 16 functioning as a mask.
Thereafter, select portions within the active layer 14 are doped with impurities to define a source region 14S and a drain region 14D. A channel region 14C located between the source region 14S and the drain region 14D is also defined.
Note that it is necessary to clean the exposed portion of the substrate 100 before depositing a gate insulating layer 15 on the substrate 100. The cleaning process prevents foreign substances from contaminating the interface between the gate insulating layer 15 and the active layer 14. The cleaning process involves wet cleaning the substrate 100 with an HF solution before depositing the gate insulating layer 15 onto the substrate 100.
Referring to FIG. 1D, a protective layer 17 is deposited on the entire surface of the substrate 100 including the gate electrode 16. The protective layer 17 is then patterned and etched to form contact holes. The contact holes expose the second source metal line 12S, the source region 14S, the drain region 14D, and the second drain metal line 12D. Next, a transparent conductive layer is deposited on the exposed portion of the substrate 100. The transparent conductive layer is also patterned and etched, and forms a first metal line 18-1, which connects the second source metal line 12S with the source region 14S, and a second metal line 18-2, which connects the second drain metal line 12D with the drain region 14D.
As noted previously, in the prior art, a cleaning process with an HF solution is performed on the exposed portion of the substrate 100 before the gate insulating layer 15 is deposited onto the substrate 100. However, because the first source metal line 11S and the first drain metal line 11D are made from a low resistance metal such as aluminum, the source and drain metal lines are damaged by the cleaning process because of the strong etching properties of the HF cleaning solution.
To overcome the problems described above, preferred embodiments of the present invention provide a method for manufacturing a thin film transistor where a protective layer is provided to protect source and drain metal lines, which are made from a low resistance metal such as aluminum, from being damaged during a cleaning process.
According to a first preferred embodiment of the present invention, a method of manufacturing a thin film transistor includes the steps of forming source and drain metal lines on an insulating substrate, forming a first protective layer covering the source and drain metal lines, sequentially depositing a buffer layer and a semiconductor layer on the first protective layer, forming an active layer by patterning and etching the semiconductor layer, etching the buffer layer using the active layer as a mask, performing a cleaning process before depositing a gate insulating layer on an exposed entire surface of the substrate, forming a gate insulating layer and a gate electrode on the active layer, forming source and drain regions in the active layer by doping the active layer with impurities using the gate insulating layer as a mask, forming a second protective layer covering the exposed surface of the substrate including the gate electrode, forming contact holes in the first and second protective layers so that the source and drain metal lines and the source and drain regions are exposed, and forming a first metal line connecting the source metal line and the source region, and forming a second metal line connecting the drain metal line and the drain region.
According to a second preferred embodiment of the present invention, a method for manufacturing a thin film transistor includes the steps as described in the first preferred embodiment except that the source and drain regions are formed before forming the gate electrode.
Other features, elements and advantages of the present invention will be described in more detail in the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.